Herbivore effect traits and their impact on plant community biomass: an experimental test using grasshoppers

Female grasshopper (Chorthippus biguttulus)

Herbivorous arthropods, such as insects, may play an important role in regulating plant diversity and ecosystem functioning (e.g. nitrogen cycling). But it is unclear which mechanisms drive plant – arthropod interactions and ultimately arthropod effects on plant communities. Various herbivore characteristics have been assumed to determine herbivore impact on plant communities. For instance, herbivore body size has been proposed as a key trait determining the quantity of biomass consumed. But its effect might be modulated by herbivore food preferences (i.e. herbivore feeding niche). In this case, herbivore chemical traits (e.g. carbon to nitrogen ratio) or biomechanical traits (e.g. mandibular traits; biting strength) have been hypothesised to be related to herbivore feeding niche. Yet, how functionally contrasted herbivores may impact plant community biomass in real field conditions, and what is the relative importance of different herbivore traits, has never been experimentally tested.

This study set up a cage experiment in a species-rich grassland and tested how grasshopper traits may explain their effect on plant biomass. Six grasshopper species were selected because they show contrasted traits and feeding niches.

Grasshopper impact ranged from 0% up to 60% depending on the species considered. By comparing the relative importance of multiple interacting grasshopper traits, biting strength appeared to be a key trait determining grasshopper feeding niche and impact on plant biomass. Importantly, the study demonstrated that only two simple plant traits (Carbon to Nitrogen ratio and leaf dry matter content) well predicted grasshopper feeding niche. For instance, herbivores with strong mandibular strength preferentially chose tough leaves while herbivores with weak mandibular strength selected opposite plant attributes.

This study provides a first experimental test of the relationship between herbivore traits and their niche, which in turn determines their impact on plant community biomass and ultimately on ecosystem functioning. It also contributes to the development of a trait-based approach in a multitrophic perspective and shows that simple traits can predict the intensity of trophic linkages and herbivore effects at the level of the entire plant community.



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